1. Field of the Invention
This invention relates to a rectangular prism utilized in an optical instrument such as a viewfinder.
2. Related Background Art
What poses a problem in the viewfinder optical system of a photographic camera, a video camera or the like is the processing of ghost light created in this system. The main cause of the ghost light is the shape of a prism incorporated in the system.
The shape of a rectangular prism heretofore used in a portion of a viewfinder optical system and the optical path therein are shown in FIG. 1 of the accompanying drawings. FIG. 1 shows a rectangular prism in which an entrance surface 10 and an exit surface 20 coincide with the same plane and which has reflecting surfaces 30 and 40 orthogonal to each other. An effective light beam incident from the entrance surface 10, as shown by light rays 1 indicated by solid lines, is reflected by the first reflecting surface 30 and the second reflecting surface 40 and emerges from the exit surface 20 in the direction opposite to the direction of the incident light beam. However, in the actual state of use in which the rectangular prism is incorporated in the optical system, light rays enter in various directions and therefore, a light ray which may become ghost light also enters at the same time. Such light ray, like a light ray 2 indicated by broken line, has always had the risk of being reflected by the first reflecting surface 30 and totally reflected by the entrance surface 10, and thereafter being reflected by the second reflecting surface 40 and emerging from the exit surface 20. If such a light ray 2 emerges from the exit surface 20, ghost or flare will be created to deteriorate the performance of the entire optical system remarkably. Therefore, in the design of an optical system using a prism, the greatest circumspection has been necessary in deciding the shape of the prism. As a technique therefor, a U-shaped groove as shown in FIG. 2 of the accompanying drawings has been formed in the boundary between the entrance surface and the exit surface present on the same plane and the wall of the groove has been subjected to the treatment of painting the wall black. By the inner surface of the groove being painted black, ghost light is intercepted by the groove as indicated by broken line 2 and cannot arrive at the second reflecting surface 40.